1. Field of the Invention
The present invention relates to cathode ray display devices and in particular to the preservation of predetermined relationships between video signals applied to the devices and electron beam currents produced therein.
2. Description of the Prior Art
Conventional image display apparatus, such as color televison receivers employing multibeam cathode ray display devices, are provided with potentiometers for adjusting the biases and the gains of video amplifiers driving electron gun cathodes producing the electron beams. These potentiometers are factory adjusted to present neutral gray images when a video signal representing a gray image of any intensity is applied to the video amplifiers. The bias adjustments, made while a video signal representing a low intensity gray image is applied, establish the output voltages of the video amplifiers at magnitudes which cause the production of low magnitude electron beam currents having predefined ratios resulting in a low intensity gray image. At other image intensities, the ratios of these beam currents must be maintained to achieve gray scale tracking. The gain adjustments, made while a video signal representing a higher intensity gray image is applied, set the gains of the amplifiers to compensate for differences in the transconductances of the respective cathodes driven thereby and establish individual transfer functions such that the predefined ratios of the beam currents are maintained at all intensities.
Adjusting the biases and gains of the video amplifiers, as described above, accomplishes more than the production of neutral gray images. It also causes the beam currents to be maintained at predefined magnitudes for any given hue, saturation level and intensity represented by a video signal applied to the video amplifiers. Maintaining gray scale tracking is essential for the accurate representation of color images.
Although control of the biases and gains of the video amplifiers is an effective manner of achieving and maintaining gray scale tracking, control by manual adjustment of potentiometers suffers a major disadvantage--lack of permanency. The electrical characteristics of the cathode ray devices drift with age, change with cathode temperature and are susceptible to changes caused by mechanical shock. Also, the output voltages of the video amplifiers vary with temperature and the supply voltage. Thus, continual bias and gain readjustment is necessary if degradation of color quality is to be avoided.
Continual bias and gain readjustment is also necessary in single beam image display apparatus such as monochromatic television receivers. Here readjustment is necessary to maintain initially established relationships between video signal magnitude and image intensity.
Circuitry is known for automatically adjusting beam current in a CRT and thus eliminating the need for manual adjustments. The known circuitry works upon the premise that the beam current produced by each cathode in the CRT is equal to the cathode current itself. This is not realistic because it fails to account for leakage current which often exists in CRTs. The magnitude of this leakage current, which varies with the temperatures and voltages of the CRT device elements between which it flows (typically, the cathode, the heater filament and a control electrode adjacent to the cathode), can be substantial with respect to the beam current, even for high magnitude beam currents. The error in ignoring leakage current becomes most apparent at low beam currents, however, because the leakage component of the cathode current increases as the beam current is decreased. At the lowest beam currents the leakage current component can become orders of magnitude larger than the beam current component. This large error at low beam currents is particularly objectionable in the display of color images because the eye is most sensitive to color abnormalities at low image intensities. It is also objectionable in the display of monochromatic images because the eye is most sensitive to intensity abnormalities at the dark level. Thus, beam current control circuitry which does not take such leakage current into account cannot adequately maintain gray scale tracking.